Physics

Photoelectric Effect

PhET Virtual Experiment

Procedure:

1. http://phet.colorado.edu/sims/photoelectric/photoelectric.jnlp

a. Threshold Frequency and Work Function

b. Define:

i. Threshold Frequency –

ii. Work Function –

c. Explore: Under Graphs, place a check next to Electron Energy vs. Light Frequency. Use this graph to determine the threshold frequency and work function for each sample.

Sample Threshold Frequency

( x 1015 Hz)

Work Function

(eV)

Sodium
Zinc
Copper
Platinum
Calcium
?????

d. Using any one of the samples, determine the slope of the graph. Show your work below. Compare the magnitude of the slope to Planck’s constant.

2. What role does light intensity play in the Photoelectric Effect?

a. Key Concepts:

i. What is another word for the “intensity” of light?

ii. Electric current (covered later in our course) refers to the rate at which electrons flow. In this case, the current is increased by causing more electrons to flow.

b. Explore: Choose any one of the samples to use for this experiment. Under graphs, make sure that both Electron Energy vs. Light Frequency and Current vs. Light Intensity are checked.

i. Below the Threshold Frequency –

· Adjust the frequency of the incoming light so that it is below the threshold frequency. Now experiment with the intensity slider. Are there any intensity levels that produce electric current?

ii. Above the Threshold Frequency –

· Adjust the frequency of the incoming light so that it is above the threshold frequency. Now experiment with the intensity slider.

a. How does the intensity of the light affect

i. The number of electrons that flow

ii. The speed at which the electrons flow

b. Are there any intensity levels for which there is no electrical current?

3. Stopping Voltage

a. Define: Stopping Voltage –

b. Explore: Choose any one of the samples to begin this experiment. Under Graphs, place a check next to Current vs. Battery Voltage.

i. Adjust the frequency of the incoming photons to 152 nm.

ii. Start with the battery voltage set at zero.

1. What happens when the voltage is moved to a positive value?

2. What happens when the voltage is moved to a negative value

iii. Use the graph to estimate the stopping voltage for each sample.

Sample Stopping Voltage

(V)

Sodium
Zinc
Copper
Platinum
Calcium
?????

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